Athletic shoes are known to be more comfortable and lighter in weight than traditional footwear formed of a single “cup” sole. This is because the cup sole has had to provide, by itself, a number of different properties, including hardness or anti-wearing capability, shock absorption and spring, and optimizing these properties typically requires trade-offs.
The athletic shoe provides a two-part sole employing an outsole and a separate midsole overlying the outsole. By essentially splitting the cup sole into two parts, it is possible to tailor each part more particularly for its function and achieve increases in performance and comfort along with decreases in weight. For example, the outsole can be tailored for anti-wearing capability, while the midsole can be tailored for shock absorption, spring, and comfort. The outsole and midsole can be provided with different densities as well as other differing material characteristics.
There have been efforts to form a cupsole mimicking an outsole/midsole design by forming the cupsole in a two stage molding process. However, the material limits imposed by such processes have deprived them of achieving complete success. There remains a performance and weight advantage in the outsole/midsole design.
In parallel with the trend toward performance oriented footwear, there has been a trend in the workplace away from heavy industry and toward light industry. The latter generally requires lighter gear and apparel, including shoes, and workers to a greater extent can select for the workplace the footwear that they prefer, which is often the same footwear that they choose to wear on the street. For example, it can be seen that the old-style, typically heavy, safety shoe is required to a greater extent where workers handle heavy equipment and move heavy loads in environments that are relatively hostile. These requirements are significantly diminished in the relatively benign environments found in the more office-like factories employed for light assembly. As a particularly important example, workplace safety concerns in the electronics industry are lessened because workers are not typically exposed to the risks of operating heavy machinery and handling heavy parts. It is very common in this industry for workers to wear the same shoes during work that they wear for purposes of leisure.
Integrated electronic components have always been susceptible to damage from the electrostatic charges that normally build up on a workers'body as the worker moves about. These charges can be as high as 20,000 volts and may be conducted into the components so as to arc across individual transistors inside, permanently damaging them. This risk continues to increase as the spacing between current carrying paths on the integrated circuits become closer together, in order to increase the density of electronic components on the integrated circuit. Because of this, it is now mandatory in electronics assembly to provide for some means for draining or dissipating static charge from workers through a controlled resistance that, although quite high, has been found to be effective for this purpose.
The prior art has frequently recognized the need to provide means for controlling electrostatic discharge integrated with a worker's shoe. Edwards, U.S. Pat. No. 4,785,371 as well as others provide such means in footwear employing cup sole construction. Cheskin, U.S. Pat. No. 5,448,840 proposes to provide ESD in an athletic shoe; however, the proposal requires providing a conductive integral extension of a ground contacting outsole which extends from a peripheral edge of an outsole upwardly around the edge of other shoe sole components and is integrally attached to the inside or outside surface of the upper of the shoe such that it makes substantial contact with the foot of the wearer. As the conductive extension is formed of the same material as the outsole, this has a disadvantage of increasing the relative amount of outsole material required to form the shoe, defeating to some extent the gains achieved by splitting the cup sole into an outsole and a midsole. An additional disadvantage of the conductive integral extension is that it forms an additional structural member that impacts and alters the desired characteristics of the shoe. Cheskin recognizes, however, that the materials of which a sockliner, insole, midsole and outsole are comprised are typically different and separate, so that creating an effective electrically conductive path between the bottom of the wearer's foot and the outsole is difficult.
It may be noted that electrostatically dissipative shoes are commonly rated as to their resistive characteristics. A so-called “type 1” rating indicates a shoe with an overall resistance, from a topmost surface of the shoe sole to a bottommost surface of the shoe sole, in the range of 1–100 megohms, and a “type 2” rating indicates an overall resistance in the range 1–1000 megohms.
The assignee of the present invention has manufactured and marketed athletic shoes having ESD characteristics that have provided all of the advantages of athletic shoe structure and represented a significant advance over the then-current prior art. However, further improvements in the reliability and cost-effectiveness of the ESD characteristics remain desirable. Accordingly, there is a need for an electrostatically dissipative athletic shoe that provides for increased control of ESD characteristics as well as decreased cost of manufacture.